Painless Death, a 1st level Cleric spell from Ghostwalk.
It can only be cast on a willing living creature, but it automatically kills them, no save, no SR, no nothing.
And it is not a [Death] effect.
If I remember correctly, you should be able to use Ocular Spell with this (to bypass the Target line of the spell.), or alternatively Charm an opponent, or, hell, use the False Theurgy skill trick, combo'd with a bluff check, to make someone believe that you are casting a Cure Light Wounds or something.
It is also a Touch Range spell, so there is always the fun of delivering it with a familiar.